Ultra high speed data links will become wide spread with the deployment of broadband switched digital networks and services, e.g., teleconferencing, video telephony, and computer services. The key hurdle in the commercial development of these networks is the availability of cost effective photonic technologies that will speed up the generation, transmission and processing of these vast amounts of data. Present state of the art optical communications and signal processing rely on a wavelength division multiplexed (WDM) or a time division multiplexed (TDM) hardware platform.
Thus, hardware based on multiwavelength optical signal sources capable of generating ultrashort and highly synchronized picosecond pulses are of great interest in novel photonic networks utilizing combined optical wavelength division multiplexed (WDM) and time division multiplexed (TDM) data formats. To date, multiwavelength generation has been demonstrated by either using spectral filtering of femtosecond optical pulses, or semiconductor laser based devices. See Morioka et al., "Multi WDM-Channel pulse generation from a Single Laser Source utilizing LD-pumped Supercontinuum in Optical Fibers", PhotonTech. Lett., vol.6, no.3, 365.about.368, 1994; and Nuss et al. "Dense WDM with Femtosecond Laser Pulse ", IEEE/LOS 1994 Annual Meeting, Boston, Mass., 1994.
In these approaches, the main idea is to generate femtosecond optical pulses at a low channel pulse rate. These approaches generally suffer from limited wavelength channels, the need of high power laser source, system complexity, and low single channel rates.
Furthermore, there have been various patents of general interest in this area that also fail to overcome the problems described above. U.S. Pat. No. 4,435,809 to Tsang et al. describes a passively mode locked laser having a saturable absorber that only has a single wavelength operation mode, with multiple longitudinal modes. U.S. Pat. No. 4,446,557 to Figueroa describes a mode-locked semiconductor laser with tunable external cavity where a user adjusts the cavity length which modifies the longitudinal mode spacing to generate a single wavelength output. U.S. Pat. No. 5,115,444 to Kirkby et al. describes a multichannel cavity laser where each wavelength is generated from a common cavity with each wavelength experiencing a different optical path length. Simultaneous generation of each wavelength is not feasible since the gain competition in the final optical amplifier stage will complicate and prevent simultaneous multiwavelength generation. U.S. Pat. No. 5,228,050 to LaCourse et al. describes an integrated multiple-wavelength laser array, each wavelength having its own cavity that is length adjustable to allow lasing at different wavelengths, and requires an array of lasers for the multiple wavelength generation. U.S. Pat. No. 5,319,655 to Thornton describes a multiwavelength laterally-injecting type lasers which requires the sources to be precisely aligned to one another. U.S. Pat. No. 5,524,012 to Wang et al. describes a tunable, multiple frequency laser diode that uses a multistripe semiconductor laser array to generate several wavelengths, and requires a grazing incidence angle on the diffraction grating. Using the grazing incidence angle prevents simultaneous wavelength generation. U.S. Pat. No. 5,524,118 to Kim et al. describes a wavelength-varying multi-wavelength optical filter laser using a single pump light source, which requires using an erbium doped fiber amplifier. U.S. Pat. No. 5,561,676 to Goldberg describes a compound-cavity high power, tunable modelocked semiconductor laser, that generates a single wavelength output that does not allow for multiple wavelength generation.
Another problem with multichannel generation from femtosecond lasers is that the multiple channels are generated by spectrally filtering the laser output after the optical pulse is generated. This is inefficient because the filtering process eliminates, or throws away, energy that was used in making the optical pulse.
In approaches relying on the spectral filtering of super-continuum generation, some wavelength channels may experience excess spectral incoherence, rendering them useless for data transmission.